Ceramic substrates with good mechanical and electric characteristics are used in a variety of recent electronic parts. For those, multi-layered ceramic substrates, which comprise a plurality of ceramic sheets with electrodes and circuit patterns mounted thereon and in which the inner electrodes and circuit patterns are connected with each other via holes formed through the ceramic sheets, are widely used in order to increase packaging density.
One conventional method for producing such multi-layered ceramic substrates having inner electrodes and circuit patterns connected with each other by via holes comprises the following steps (1) to (4):
(1) First, holes for "via-holes" are formed through ceramic green sheets, using a drill or punch.
(2) Next, the thus-formed holes are filled with an electroconductive paste or metal powder.
(3) Next, an electroconductive paste is applied over the surface of each ceramic green sheet through screen printing or the like, to thereby form an electrode and/or a circuit pattern thereon.
(4) Last, a plurality of those ceramic green sheets are laminated under pressure, cut into predetermined sizes, and baked, whereby the ceramic green sheets are sintered while the electroconductive paste or metal powder existing in the via-holes is also sintered, and hence the inner circuit patterns are electrically connected with each other.
In that conventional method, copper is popularly used as the electroconductive material for forming the via-holes, since it has a small specific resistivity, it hardly migrates through the laminated sheets and it is inexpensive. For example, generally used therein is an electroconductive paste prepared by mixing and dispersing copper powder in an organic vehicle comprising a resinous component such as ethyl cellulose. As in the above, the ceramic green sheets and the metal existing in the via-holes are simultaneously baked in the baking step in the conventional method. In this method, therefore, any short or excessive filling of the electroconductive paste into the via-holes and any difference in the degree of contraction between the electroconductive paste and the ceramic green sheets being baked simultaneously will produce cracks of the electroconductive metal (formed as a result of sintering of the electroconductive metal powder in the paste) in the via-holes and/or cracks of the baked ceramic sheets themselves. Those cracks of the electroconductive metal in the via-holes and the ceramic sheets cause the via-holes to fail to have good electroconductivity and make the resulting ceramic laminate substrate have structural defects, resulting in that the reliability of the ceramic laminate sheet is lowered.